Discussion
The Greenhill formula of rate of twist is based on consideration that specific gravity of bullet is generally :
Answer: D
The Greenhill formula assumes a bullet of specific gravity 10.9.
Sir Alfred Greenhill's rule estimates the twist needed to stabilise a bullet:
\[ \begin{aligned} &\text{twist (calibres per turn)} \\ &= \frac{150}{\text{length in calibres}} \end{aligned} \]
The constant 150 was worked out for a lead-cored bullet with a specific gravity of about 10.9, close to that of lead. For bullets of a different density the rule has to be corrected by the square root of the ratio of densities.
7.9 is about the specific gravity of steel and 8.9 of copper, and 9.9 is not the assumed value either.
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